When a well is drilled and steel casing is placed, cement slurry is pumped into the annular space between casing and formations. The primary objectives of cementing are to provide mechanical support for the steel casing string and zonal isolation between earth strata or formations. Multiple-stage casing and cementing operations are common procedures to establish pressure barriers during drilling a well. It allows the use of heavier drilling muds in drilling deeper sections without damaging or fracturing the shallower formations due to hydrostatic pressure gradient. An ideal cementing job would fill the casing and formation annulus completely with cement. Potential issues encountered in cementing operations are fluid filled channels within the cement sheath and fluid contaminated cement due to incomplete replacement or sweep of drilling mud with cement slurry. Zonal isolation assessment is a critical aspect of well integrity tests to ensure hydrocarbon production in a safe manner. Cement evaluation measurements are relied upon to demonstrate fluid cross flow is not expected from unwanted zones, i.e. zones other than the producing intervals. This invention relates to in situ evaluation of cement quality between steel casing and formations in a wellbore.
Gamma ray density logging technology is a well-known art to provide formation density and porosity data in petrophysical analysis and formation evaluation. A nuclear density log provides volumetric density measurements of the wellbore and surrounding media. In theory, a nuclear density tool may be constructed with a gamma ray source and one gamma ray detector. The bulk density response in a homogeneous and infinite medium is calibrated as a function of detector count rates. The calibration is normally performed in rock standards with known density value in laboratories. A unique calibration converting count rates to density values is assigned to every density logging tool. Density tools, in practice, have multiple detectors in place in order to provide accurate density measurements in layered media, such as mud cake, casing, and cement.
Existing density tools are designed to optimize sensitivity to the density in the formation rather than the density in the mud cake, casing, or cement. Interpretation methods are devised to remove mud cake, standoff, casing and cement effects as unwanted environmental effects. Shown in FIG. 1 is the traditional “spine-and-ribs” response plot of a two-detector density tool to remove the mud cake effect in an open hole. (Figure from Ellis and Springer, Well Logging for Earth Scientists, Springer (2007)) Mud cakes are solid and fine filtrates in drilling mud accumulated on the wellbore wall as a result of fluid loss to the porous rocks. The two-detector based spine-and-ribs method allows the accurate interpretation of formation density in the presence of a mud cake layer or standoff. The mud cake density is typically lower than formation density. When heavy muds such as barite mud are used, mud cake density value is greater than formation density. Interpretation algorithms using spine-and-ribs schemes are capable of producing formation rock density by correcting the mud and standoff effects. Existing density tools are seldom used in wells that are completed with casing and cement and are not capable of providing accurate formation density measurements in those circumstances. This is because the spine-and-ribs method does not remove the large casing and cement effects. These tools and the associated algorithms are not capable of accurately measuring cement properties in cased wells as these tools are not optimized to measure material properties other than mud cake.
To use FIG. 1, the count rate from a detector located close to the source (short-spaced detector) is plotted against the count rate from a detector located farther from the source (long-spaced detector). FIG. 1 then gives a correction to compensate the measured formation density for the effects of the mud cake. If there is no mud cake present between the logging tool and borehole wall, the short and long-spaced detector response would follow the primary response curve called the spine line. If there is mud cake, the detector response would follow one of the departure lines or rib lines depending upon the mud cake properties. The formation density is the long-spaced detector density plus the density correction which equals to density difference along the rib lines.
There are primarily two types of gamma ray density tools. Shown in FIG. 2 is a schematic of a typical wireline density tool which is used to acquire formation density data after a well is drilled. The typical tool configuration consists of a gamma ray source, a long-spaced detector located at approximately 16 inches from the source, a short-spaced detector located at approximately 8 inches from the source, and an optional backscatter detector at about 4 inches from the source. The back scatter measurement allows a correction to be made for borehole size irregularities such as rugosity and washouts. These detector-to-source spacing values are optimized to maximize the measurement sensitivity to the formation density and minimize the near-borehole effects due to the presence of mud cake and casing/cement. In another words, the detectors are placed at larger distances from the source, so that the mud cake, casing and cement investigation volumes account for a small portion of the total investigation volume. The tool diagrammed in FIG. 2 has one gamma ray source and two or three gamma ray detectors. The interpretation algorithms are built on spine-and-ribs based methods:ρb=ρLS+ΔρWhere ρb is formation density, ρLS is long-spaced density density, and Δρ is the mud cake density correction which is a function of long-spaced and shorted spaced density measurements:Δρ=f(ρLS,ρSS)where ρLS and ρSS are long-spaced and short-spaced densities.
It is most commonly run in open holes before steel casing and cement are placed in a wellbore. However, it is also used occasionally to log cased wells to acquire formation density logs, with much reduced data precision and accuracy.
Shown in FIG. 4 is a schematic diagram of a typical logging-while-drilling (LWD) density tool which is used to acquire formation density data while a well is being drilled. The LWD tool module is mounted on the drill pipe and is operated above the bottom-hole-assembly (BHA). When a well is drilled, a rotating BHA enables the LWD density tool to record azimuthal density image data. The LWD tool is integrated with the BHA and is not designed for cased wells density logging. A density image example is shown on the left track in FIG. 3. The right track shows a density log made by a wireline tool with no azimuthal dependency.
Following is a partial list of publications on density logging techniques.
U.S. Pat. No. 3,815,677 (“Method for operating in wells”) describes running an open hole sonic log and a cased hole neutron log to detect fluid channels in cement. Azimuthally oriented nuclear density logs are also run to detect fluid-filled channels. The oriented density scan is plotted and the density variations in the plot provide indications of channels in the cement. The disclosed method of fluid-filled channel detection is qualitative in nature.
U.S. Pat. No. 5,627,368 to Moake (“Four-detector formation-density tool for use in cased and open holes”) discloses a nuclear density tool design with four detectors to provide measurements of casing and cement weight and thickness as well as formation density. It teaches a method for measuring the thickness of the cement, and a method for measuring the density of the cement. There is no disclosure of a tool design with azimuthal capability for measurement around the wellbore to generate cement density and thickness maps.
U.S. Pat. No. 6,781,115 (“Subsurface radiation phenomena detection with combined and azimuthally sensitive detectors”) discloses a nuclear tool design with a detector system that has azimuthal sensitivity by placing multiple detectors at different azimuths. Unlike the present invention, these detectors are not placed on articulated pads to accommodate varying casing size.
U.S. Patent Application Publication No. 2008/0061225, “Logging tool for determination of formation density (Embodiments),” by Orban et al., discloses a logging tool design with one or more detectors and a rotating source. Azimuthal formation density measurements are provided by, for example, collimated shielding placed around the rotating source. The multiple sensor pad configuration of FIG. 13 generates images with gaps. A rotating source and detector configuration to provide continuous borehole scan is not disclosed.
Patent application publication WO 2011/152924, “System and method for generating density in a cased-hole,” describes a method and device for cased hole density logging. The objective is to find formation density, and interpretation algorithms are disclosed for doing that, and for making corrections for the effects of casing and cement. No disclosures are made regarding azimuthal measurements.
Commercially available logging tools/products offered by oil service companies include:
Cased Hole Analysis Tools (CHAT) by Voltage Wireline Inc., and the Three detector Litho-Density tool (TLD) by Schlumberger. With adaptations as taught by the present disclosure, these tools could provide qualitative cement density and thickness measurements, but their design objectives are for formation rock density measurements and not for accurate measurements of cement properties. Nor do they have azimuthal sensitivity to scan the wellbore.